1. Field of the Invention
The present invention relates to an image synthesizing apparatus that synthesizes images by keying.
2. Description of the Related Art
At a television channel or an image editing studio, an image synthesizing apparatus called an “effect switcher” is used to apply special effects in a process called “keying”. Keying refers to a process that superimposes a key fill signal (an image signal to be superimposed onto a background image as the foreground) onto the background image (i.e., the main image signal that makes up a screen) in a region designated by a key source signal (i.e., an image signal in which a part where the signal level reaches a threshold or higher designates a region for superimposing an image).
FIG. 1 is a diagram showing a keying circuit in a typical effect switcher. The keying circuit includes four pairs of a key image selecting switch 51 and a key processing circuit 52 (such pairs are called “keyers”), an image synthesizing circuit 53, and a background image selecting switch 54.
In each keyer, the key image selecting switch 51 is a switch for selecting an image signal to be used for keying out of image signals inputted on input lines I1 to I9 from external video cameras, VTRs (Video Tape Recorders), and the like, and is constructed of a switch 51a for selecting a key fill signal and a switch 51b for selecting a key source signal.
The key fill signal and key source signal selected by the key image selecting switch 51 are sent to the key processing circuit 52. Based on various parameters, the key processing circuit 52 adjusts and processes the key fill signal and the key source signal to become suitable for keying. For example, values for adjusting the density of the key fill signal relative to the background image, values for adjusting the threshold of the key source signal, values for adjusting the position of the key source signal, values for adjusting a reduction ratio of the key fill signal, and adjustment values relating to a boundary line with the background image are used as the parameters.
The key fill signal and the key source signal that have been adjusted and processed by the key processing circuit 52 in each keyer are sent to the image synthesizing circuit 53. A background image that has been selected by the background image selecting switch 54 out of the image signals inputted on the input lines I1 to I9 is also sent to the image synthesizing circuit 53. The image synthesizing circuit 53 carries out a process on the key fill signals and the key source signals from the respective keyers to superimpose the key fill signals as foreground images on the background image using the key source signals as key values (alpha values for density when synthesizing images).
Only one pair of a key fill signal and a key source signal can be sent from each key processing circuit 52 to the image synthesizing circuit 53. Accordingly, only one foreground image can be superimposed on the background image at a time by a single keyer.
The present applicant has already proposed an invention that enables the operator to easily select a pair of a key fill signal and a key source signal from a plurality of candidates. However, according to such invention, only one pair of a key fill signal and a key source signal can be selected at a time. Accordingly, only one foreground image can be superimposed on the background image at a time by a single keyer (see Japanese Patent Application Publication No. 2002-185851).
With a television broadcast, there are many cases where a plurality of foreground images are simultaneously superimposed onto a background image. For example, the present time is displayed at an upper right position on the screen, a program logo is displayed at an upper left position on the screen, and explanatory subtitles are displayed at a lower position on the screen. In this case, with the effect switcher shown in FIG. 1, each keyer sends a pair of a key fill signal and a key source signal for superimposing one foreground image to the image synthesizing circuit 53. One keyer sends a key fill signal and a key source signal for superimposing the present time to the image synthesizing circuit 53. Another keyer sends a key fill signal and a key source signal for superimposing the program logo to the image synthesizing circuit 53. Yet another keyer sends a key fill signal and a key source signal for superimposing the explanatory subtitles to the image synthesizing circuit 53.